Sodium and water regulation by the kidney plays a key role in hypertension and can be significantly compromised by pathways of oxidative stress. Two tubular elements are of major importance in establishing Na+ homeostasis and both are known to participate in salt-sensitive forms of hypertension, the medullary thick ascending limb of Henle (mTAL) and the aldosterone sensitive distal nephron (ASDN). The mTAL of SS rats produces excess ROS and the chronic intramedullary infusion of catalase, a scavenger of H2O2, reduces salt- induced hypertension nearly 50% in SS rats. Conversely, medullary infusion of H2O2 to normal rats reduces MBF and Na+ excretion resulting in a salt-sensitive form of hypertension. SS rats fed a high salt diet also exhibit greater expression and activity of ENaC in the ASDN segments leading to greater reabsorption of Na+ and enhancement of salt-induced hypertension. The major source of ROS and H2O2 in the kidney is NADPH oxidase but the roles of specific Nox isoforms such as Noxs 1, 2 and 4 and the mechanisms whereby they affect renal function have not been well elucidated. The most abundant isoform in the kidney is Nox4 which is unique in that it releases predominantly H2O2. Yet no studies have been carried out to determine the role of Nox4 in Na+ homeostasis and hypertension. We hypothesize that Nox4 plays a dominant role in determining blood pressure salt-sensitivity in the SS rat in two ways: 1) By excess production of H2O2 in the renal outer medullary thick ascending limbs of Henle (mTAL) which diffuses to surrounding vasa recta (VR) pericytes causing constriction and reduction of MBF; 2) Through H2O2-mediated increases of ENaC activity in the ASDN. To explore the role of Nox4, we have created a novel rat model with a null mutation of Nox4 in the SS rat. We will compare the responses of this mutant rat, SSNox4-/-, to those of the SS rat in four Specific Aims: 1- Determine physiological consequences of a null mutation of Nox4 in SS rats (SSNox4-/-) upon whole kidney function (MBF and GFR), renal oxidative stress, pressure-natriuresis, salt-induced hypertension and renal injury. 2-(New Aim) Determine the extent to which the reduced renal injury in SSNox4-/- rats is a consequence of a lower renal perfusion pressure versus an inherent intrarenal reduction of ROS production (servo-control of renal perfusion pressure studies). 3-Determine if Nox4 is importantly involved in H2O2 production in mTAL in response to increased luminal Na+ delivery and whether H2O2 can diffuse from mTAL to constrict surrounding VR. 4-Determine if production of H2O2 and ENaC expression/activity in ASDN of SS rats is Nox4-dependent. Studies are multiscale in design ranging from intracellular to those utilizing chronically instrumented rats which monitor changes in MBF and GFR over several weeks. The results are expected to greatly enhance our understanding of the role of Nox4 in renal function and lead to novel ways to target pathways of oxidative stress in the treatment of hypertension and renal disease.